This invention relates to concrete, which is a combination of a hydraulic cementing substance, aggregate, water, and, often other substances to impart specific properties to the concrete.
When concrete is poured it is typically in a watery or flowing state which allows the concrete to be spread evenly over floors. After a period of time, varying with the mixture of the concrete, the temperature, and the moisture availability, the concrete attains a workable plasticity which permits the surface of the concrete to be formed and to retain a finish. Typical finishing means include troweling, rubbing, or brushing. Applying the desired surface texture is called "finishing" the concrete, and may involve repeated steps to sequentially refine the surface finish.
After the concrete is finished, it is allowed to stand for a period of time during which the concrete cures to obtain its well-known, rock-like hardness. The curing or setting time depends on the moisture available, the temperature, and the specific additives added to the concrete to affect the curing time. As the concrete cures it undergoes thermal stresses causing the concrete to expand and contract in various manners depending on the shape and thickness of the concrete, and the type of concrete. These thermal stresses can cause cracking. The fully cured and hardened concrete also expands and contracts due to temperature changes with the result that cracks form in the concrete.
It is common practice to provide slots or grooves at predetermined intervals in the concrete. If the grooves extend all the way through the concrete, they can act as an expansion or contraction joint to help prevent cracking of the concrete. If the grooves are only on the surface of the concrete, then the grooves cause the cracks to form along the grooves so that they occur at regular intervals and are not visible. The grooves, but not the cracks, are visible.
One advantage to placing the grooves in the soft, concrete is that a weakened plane is provided by the groove and that weakened plane is now installed before the concrete starts to cure and shrink. The concrete slab will typically seek out the weakened plane to crack in, if the plane is prematurely there.
Presently, these grooves are provided by forming or grooving a slot in the concrete with a grooving trowel, while the concrete is still wet, just after pouring. This grooving is done while the concrete is very wet, and before the concrete is sufficiently hard to support a persons weight. Thus this grooving typically requires a support structure which would enable the person doing the grooving to reach the interiors of concrete slabs without placing the person's weight on the concrete. When the concrete slabs become sufficiently large, this method of providing grooves proves impractical and expensive.
This type of grooving must be done when the concrete is sufficiently wet, otherwise the grooving trowel cannot shove entrained rocks out of the way without it disrupting the surface finish on the concrete. Essentially, the concrete must be grooved just after it is has just been poured, at which time the concrete is so wet that the concrete sometimes tends to sag back together and close the groove, thus requiring repeated grooving to maintain a desired groove depth or shape.
For very large slabs of concrete, manually grooving the freshly poured concrete is impractical or very inconvenient and expensive. For such large slabs the concrete is typically allowed to harden or set. Grooves are then cut in the surface of the concrete by use of a high-powered, rotating, abrasive saw blade, often lubricated with water. The blade is typically made of diamond abrasive material and is provided with a liquid coolant and lubricant to facilitate cutting the hardened concrete.
Since these concrete cutting machines tend to be heavy, the concrete must be fairly hard in order to support the weight of the machine and operator. Further, if the concrete is not sufficiently hard when cut, these machines produce an unacceptably rough cut with a chipped or cracked surface along the groove. However, the harder the concrete, the more difficult it is to cut.
It is possible to use a hand held rotary saw as is often used in cutting lumber, but using a blade designed to cut concrete. Such saws are lighter weight, but still require hard concrete to support the operator and to provide cut grooves with acceptable smooth edges.
On an extremely hot and dry day, the concrete may be sufficiently hard to support a person's weight and not leave a permanent indentation, about twelve hours after the concrete has been poured. Typically, the concrete is not walked upon or cut until at least the next day, or about eighteen hours after the concrete has been finished.
If the concrete is cut by a conventional water lubricated diamond-abrasive saw, the earliest it can be cut is the next day after finishing (about 18 hours), and even then a unacceptable cut is typically produced as the edges of the concrete by the groove tend to chip, spall and crack.
One major problem with cutting after the concrete cures and hardens is that between the time of the initial finish and the time it become practical for a conventional concrete saw to be used, the concrete slab will have started its normal characteristic to shrink as it dries, thus causing contraction stress and invariably cracking before the sawing of contraction joints can be performed. This characteristic shrinking usually takes place somewhere between the time the initial finish is completed and before it becomes practice to put a conventional saw-cutting machine on the slab. The result is cracking of the slab before saw cutting can be initiated.
Further, cutting the hard concrete is a slow process, which is slowed still further to periodically replace the cutting blades as they abrade away. Finally, these types of machines tend to be not only bulky, but also expensive and time consuming to operate and maintain. The noise of the saw abrading the hardened concrete is also very loud and unpleasant.
There thus exists a need to provide an easier and faster apparatus and method for putting grooves in concrete before the concrete cracks.